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In Reply to: Upsampling? Interpolation? posted by Frihed89 on September 28, 2005 at 02:17:13:
When upsampling, you don't add any information. You just add samples. Why would you do that?
Figure the analog outputs of the DAC before filtering, without and with upsampling. The unfiltered analog outputs are made of a succession of staircases. The upsampled analog output has staircases smaller than the non-upsampled one. How much? Say for a CD upsampled to 384Ksamples/s from about 44KSamples/s, you have staircases 8 times smaller.
Following the DAC is a low-pass filter, whose job is to take off all artifacts above the audio band (in fact above the Niqvist frequency which is the half of the sample rate, so 22KHz) Said in another fashion, it will smooth the staircases.
Its job is just easier with the upsampled output (think of the smaller staircases) and it is a very interesting feature since you can build the filter less stringent, you can use smooth filters (bessel or butterworth) that won't put ripples in the response curve and pits and bumps in the phase response as would do a brick-wall filter (Chebitchev filter) you would have to use for non-upsampled signals.
To put it another way, for non-upsampled signals, this filter is expected to be as transparent as possible up to 20KHz, and cut everything above 22KHz (more than 60dB cut). Tradeoffs have to be made, and severe ones.
For upsampled signal, for example at 384KHz (8x upsampling), a smooth 4th order Butterworth filter (or a sinx/x) cutting 24dB/octave from about 50KHz will do the job easily without jeopardizing the audio band, and without trade-off. Further, for less money...Note for techies: I do know this explanation is not the one I should give in a digital signal processing course for MS, and I don't tell anything about spectrum folding, images and the like. It just have the advantage not to be wrong ;-)
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Follow Ups
- Re: Upsampling? Interpolation? - Jacques 03:25:32 09/28/05 (6)
- Re: Upsampling? Interpolation? - Srajan Ebaen 07:38:54 09/28/05 (5)
- Re: Upsampling? Interpolation? - morricab 05:33:06 09/29/05 (0)
- Interesting - unclestu52 12:07:34 09/28/05 (0)
- Audio Note is not filterless - Dave Kingsland 08:52:21 09/28/05 (2)
- Re: Audio Note is Now Filterless - Peter Qvortrup 08:15:49 09/30/05 (1)
- Sorry, thanks for the clarification (nt) - Dave Kingsland 09:38:08 09/30/05 (0)
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